Numerous devices already exist for limiting yaw movement.
They generally consist of lateral telescoping arms with a damping effect. These arms are mounted on either side of a coupling and connected by a ball and socket joint to the rear of the motor vehicle and by a ball and socket joint on the other end to the coupling or to the trailer.
Unfortunately, these arms limit steering angles and modify kinematics. Furthermore, they are especially cumbersome.
Other devices also exist for damping yaw movement, such as those described in Patent Nos. FR 2,796,887, FR 2,044,031, DE 25 27 673A, DE 77 18 934U and CH 675 858 A, consisting of a curved portion or a section of metallic discs and a bracket attached to the tiller of the trailer and equipped with two friction slides. These slides are pressed firmly against either side of the curved portion or the disc section by a pneumatic or elastic device which serves to brake the pivoting motions of the tiller in relation to the towing vehicle using friction and to damp the yaw movements of the trailer.
While satisfactory from the functional point of view, these existing damping systems are cumbersome and restrictive to use, since they necessitate a fairly high level of upkeep by the user.
In practice, since the friction surface is limited, the friction slides tend to wear out rapidly during use and need regular replacement in order for the device to continue to be effective.
Moreover, it is difficult for a driver to have to remember to check for wear on the slides.
In addition, damping devices of this type are located on the exterior and have no physical protection. They are exposed to the elements, incoming pollution, and attack by abrasive material such as sand, gravel, as well as liquids, oily substances, etc., which collect on the friction surfaces. All of these factors increase wear considerably on the friction slides because irregular wear translates rapidly into uniform wear.
Furthermore, with pneumatic or hydraulic systems the user must also monitor hydraulic or pneumatic pressure to ensure that the device remains reliable.
Due to the controls and maintenance requirements, the usefulness of such a device is therefore quite limited.
These prior art systems also present the problem of bulk in the area of the coupling between the motor vehicle and its trailer. They are actually quite cumbersome and must be attached in an area where numerous flexible elements and other structural connectors pass.
Yet another coupling device is described in U.S. Pat. No. 3,801,133. It comprises a ball articulation and two bars associated with the tow bar and connected to the tiller of the trailer. During yaw movements, these paired bars pivot about an axle that coincides with the axle of the ball articulation. The two bars are joined transversely by a connecting rod, the upper portion of which supports a friction plate that rubs against a friction coating covering a semi-circular extension of the main body of the coupling device connected to the towing vehicle.
When the bars pivot, a cam device causes a different force to be applied to each of these bars, thereby modifying the distribution of compression force between the friction surfaces so that the force exerted is not uniform, but is stronger on one of the bars.
This prior art device has disadvantages similar to those described previously.